1. Field of the Invention
The present invention relates generally to a method for producing a chromium containing molten iron. More specifically, the invention relates to reduction of chromium containing molten iron produced from chromium oxide, such as chromium ore, for desulphurization.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 60-9815 and Japanese Patent Second (allowed) Publication (Tokko) Showa 62-49346 disclose technologies for melting reduction of chromium oxide, such as chromium ore utilizing a top and bottom-blown converter. In the disclosed technology, chromium oxide and a carbon containing reducing agent which also serves as a heat source, are charged in molten pig iron in a melting bath. Oxygen jet is injected to the molten bath for combustion of carbon to perform reduction of the chromium oxide by the heat generated by combustion of the carbon.
In such melting reduction process, a large amount of carbon containing material, such as coal, is used as heat source and reducing agent. Such carbon containing material generally contains sulphur in a content of 0.5 wt %. Therefore, the concentration of sulphur in the molten iron increases according to the increasing amount of carbon material. The relationship between the amount of carbon material and concentration of sulphur ratio versus overall molten iron amount=% S) is shown in FIG. 6 in the accompanying drawings. Therefore, desulphurization treatment has been required after the reduction process. For example, desulphurization treatment can be performed by flux injection after tapping the molten iron from the converter. This requires an additional process for desulphurization and thus clearly lowers production efficiency.
In order to avoid the need of an additional desulphurization process after tapping, a desulphurization process is generally performed in the converter during the reduction period after decarbonization. However, such process increase the load in the reduction process to create the following problems.
First of all, during the reduction period after the decarbonization process, large amount of ferrosilicon which is known as an inexpensive reduction agent, is used for reduction. Therefore, in order to promote desulphurization, it is required to maintain basicity at a high level. Therefore, the required amount of calcium hydroxide is increased. Secondly, increasing the amount of calcium hydroxide requires the temperature of the melting bath to rise for compensation of heat and for promoting desulphurization. This accelerates damaging of the refractory in the converter wall. Furthermore, in order to maintain the oxygen potential in the melting bath during the desulphurization process, an additional amount of ferrosilicon as a deoxidation agent becomes necessary. In addition, performing desulphurization in the converter necessarily expands the process period in the converter to cause shortening of life of the refractory. Expansion of the process period in the converter also increases the amount of bottom-blown inert gas, such as Ar gas which is expensive.
Therefore, it is desirable to produce a low sulphur concentration molten iron through reduction process.